1. Field of the Invention
The present invention relates generally to a mobile communication system for high-speed packet transmission, and in particular, to an apparatus and method for scrambling a packet data channel and a packet data control channel before transmission. Further, the present invention relates to an apparatus and method for scrambling a packet data channel and a packet data control channel with a scrambling code generated using information other than control information transmitted over the packet data control channel.
2. Description of the Related Art
In general, a mobile communication system for high-speed packet transmission is divided into two systems; one system supporting only data transmission and another system supporting voice transmission as well as the data transmission. The mobile communication system for high-speed packet transmission is designed to use a high-speed packet data transmission channel for a high-speed data service. The high-speed packet data transmission channel is shared by a plurality of users (or terminals) on a time division multiplexing (TDM) basis in order to transmit data at high speed.
In the mobile communication system for high-speed packet transmission, a transmitter transmits various control information of data transmitted through the high-speed packet data transmission channel on a TDM basis, over a packet data control channel (PDCCH). The users should previously receive control information for the data containing information pertaining to a destination, a data length, a data rate and a modulation mode, among others, of the data transmitted at a specific point in time to be provided with a data service through the high-speed packet data transmission channel.
The control information for the packet data includes subpacket length information, MAC (Medium Access Control) ID, data rate, modulation mode, payload size, subpacket ID (SPID), and ARQ (Automatic Repeat Request) channel ID. In the mobile communication system for high-speed packet transmission, a transmission unit of the data transmitted through the high-speed packet data transmission channel is called a “subpacket”, and the “subpacket length information” refers to a time length of the data transmitted over the high-speed packet data transmission channel on a TDM basis. A system supporting a variable data length must transmit this information to the users. The MAC ID, an identifier for user identification, is assigned to the user desiring to receive a high-speed packet data service during system access. The “data rate” means a transfer rate of data having the subpacket length, and the “modulation mode” indicates a selected one of QPSK (Quadrature Phase Shit Keying), 8PSK (8-ary Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation) and 64QAM (64-ary QAM) modulations used to modulate the transmission data. The “payload size” refers to the number of information bits constituting one subpacket, and the subpacket ID (SPID), an identifier of each of the subpackets, is used to support retransmission. The ARQ channel ID, an identifier for supporting continuous data transmission to one user, is used in identifying a parallel transmission channel.
As described above, in the mobile communication system for high-speed packet transmission, the control information transmitted over the packet data control channel includes 2-bit subpacket length information, 6-bit MAC ID, 2-bit payload size, 2-bit SPID and 2-bit ARQ channel ID, and the data rate and the modulation mode are determined depending on the 2-bit subpacket, the 2-bit payload size and Walsh function information used for a packet data transmission channel transmitted through another channel. Thus, upon receiving a packet data control channel after being assigned MAC ID during system access, a terminal (or user) desiring to be provided with the high-speed packet data service demodulates the received packet data control channel and analyzes the MAC ID to determine whether the received packet is destined thereto. If so, the terminal demodulates the packet data channel using information on subpacket length, payload size, SPID and ARQ channel ID, acquired by demodulating the packet data control channel and information on a Walsh function used for the packet data channel. Here, information on a data rate and a modulation mode of the received subpacket is determined based on a combination of the subpacket length, the payload size and the Walsh function used for the packet data channel.
For example, the mobile communication system for high-speed packet transmission could transmit the packet data control information using two packet data control channels: a forward primary packet data control channel (PPDCCH) and a forward secondary packet data control channel (SPDCCH).
Meanwhile, the mobile communication system for high-speed packet transmission transmits user data through a structure of a forward packet data channel (F-PDCH), a high-speed packet data transmission channel, illustrated in FIG. 1. The mobile communication system scrambles coded symbols using a scrambler 102 in order to prevent other users from demodulating the user data.
Referring to FIG. 1, an encoder 101 encodes a stream of input information bits of a forward shared packet data channel and outputs coded symbols. A scrambling code generator 102 generates a scrambling code for scrambling packet data. An XOR operator 103 scrambles the coded symbols from the encoder 101 using the scrambling code. The scrambling code generator 102 and the XOR operator 103 constitute a scrambler. A channel interleaver 104 interleaves the scrambled coded symbols from the XOR operator 103 according to a predetermined interleaving rule. A puncturer 105 punctures an output of the channel interleaver 104 according to a given rule. A modulator 106 modulates an output of the puncturer 105 and outputs modulated symbols. A sequence repeater 107 repeats the modulated symbols from the modulator 106 a predetermined number of times. A symbol demultiplexer 108 demultiplexes the symbols output from the sequence repeater 107 according to the number of sub channels. A 32-chip Walsh cover section 109 spreads outputs of the symbol demultiplexer 108 with a Walsh code of length 32. A gain controller (or Walsh channel gain controller) 110 controls a gain of the outputs of the Walsh cover section 109. A Walsh chip level summer 111 sums up outputs of the gain controller 110 on a chip level.
Conventionally, the scrambling code generator 102 generates a scrambling code using the 6-bit MAC ID and the 4-bit data rate, and the XOR operator 103 XORs the coded symbols from the encoder 101 and the scrambling code. Therefore, a system whose users have no information about the MAC ID and the data rate can perform a data protection function. However, in the mobile communication system for high-speed packet transmission, all of the users can receive the PPDCCH and the SPDCCH, so one user may recognize the MAC ID and the data rate of other users. Thus, the data protection function is useless.
In addition, a base station transmits packet data control information over both the PPDCCH and the SPDCCH, and all of the terminals receive the PPDCCH and the SPDCCH, and demodulate them. That is, the packet data control information of a specific terminal is fully open to the other terminals undesirably.